1. Field of the Invention
The invention relates to an optical fiber cable. More particularly, the invention relates to an optical fiber cable using a tension member comprising organic or inorganic reinforcing fibers.
2. Description of the Related Art
An optical fiber cable is a cable comprising a plurality of converged optical fibers and has become rapidly widely used, for example, in offices and large apartments in Japan. In the optical fiber cable, tension members are used for preventing the optical fibers from being elongated to a level beyond the specified limit. Such optical fiber cables are described, for example, in Japanese Patent Laid-Open Nos. 15000/2003, 241681/2000, 107307/2003, and 23925/1999, Utility Model Registration No. 2503790, and Japanese Patent Laid-Open Nos. 199840/2000, 10382/1998, and 83385/2001.
Tension members used include aramid fiber reinforced resins (see, for example, Japanese Patent Laid-Open No. 199840/2000), glass fiber reinforced resins (see, for example, Japanese Patent Laid-Open No. 10382/1998), and steel wires coated with an enamel layer (see, for example, Japanese Patent Laid-Open No. 83385/2001).
In recent years, with the spread of FTTH, optical fiber cables have become used, for example, in main lines and branch lines, as well as in general buildings. This had led to a demand for optical fiber cables having excellent laying workability and handleabiity, that is, optical fiber cables in which the optical fibers exhibit excellent tensile and bend loss properties during handling.
Optical fibers used in this optical fiber cable include, for example, single mode fibers having a specific refractive index difference of 0.29% to 0.37% and a mode field diameter of not more than 8.8 μm. In optical fibers having the above specific refractive index difference and mode field diameter, light is surely confined in the core, and, thus, excellent bend loss properties are provided. Examples of other optical fibers include holey optical fibers comprising a clad having a plurality of holes. In the holey optical fibers, the provision of the plurality of holes in the clad increases the substantial specific refractive index difference to further ensure the confinement of light in the core. According to this construction, better bend loss properties than those of the conventional optical fibers can be realized.
This improvement in properties of the optical fibers has led to a demand for tension members having higher flexibility and excellent bending properties.
For example, tension members formed of aramid fiber reinforced resin may be mentioned as flexible tension members. AS compared with tension members formed of a glass fiber reinforced resin, however, these tension members are disadvantageous in that the minimum bending diameter (a limit diameter which causes a fracture of the tension member upon bending in an arc form) is large and the level of the so-called “curling,” which is such a phenomenon that, when the tension member is wound around a bobbin and rewound for use, the tension member is curved to a level corresponding to the winding diameter of the bobbin, is large. In the above-described Japanese Patent Laid-Open No. 199840/2000, a vinyl ester resin containing a peroxide catalyst is used as the matrix resin.
The use of a glass fiber reinforced resin instead of the aramid fiber reinforced resin is considered effective. However, none of tension members formed of a glass fiber reinforced resin have such a high level of flexibility that the minimum bending diameter D is less than 20 times the diameter d of the glass fiber reinforced resin linear material. In Japanese Patent Laid-Open No. 10382/1998, a tension member formed of glass fibers having an outer diameter d of 0.252 mm is described as a comparative example. The minimum bending curvature D is 6 to 7 mm, and D/d is not less than 20. In the working example of Japanese Patent Laid-Open No. 10382/1998, a methacrylic ester monomer-containing novolak vinyl ester resin is used as the matrix resin.
Thus, the realization of an optical fiber cable using a tension member having a higher level of flexibility using a glass fiber reinforced resin having a lower level of curling than the aramid fiber reinforced resin has been desired.
Several excellent properties are also required of the matrix resin constituting the glass fiber reinforced resin.
For example, in general, the production speed of an extra-fine tension member having a diameter of not more than 0.6 mm is not less than 1 m/min. From the viewpoint of the size of the mold and production equipment, however, the curing time cannot be brought to not less than 10 min. For this reason, the curing reaction of the matrix resin is generally carried out at an atmosphere temperature of 200° C. or below for 10 min or shorter, preferably for 5 min or shorter, more preferably 3 min or shorter, and, thus, the matrix resin desirably has a high curing speed.
Further, the viscosity of the uncured matrix resin composition is desirably low. This is because, when the viscosity of the resin composition is high, the penetration of the resin composition into gaps between glass fibers is difficult. When the glass fibers are finer, the penetration of the resin composition into yarns of a plurality of glass fibers is more difficult.
Furthermore, the use of tension members using an unsaturated polyester resin or a vinyl ester resin as the matrix resin sometimes poses a problem in post processing as an optical fiber cable. Specifically, in the step of coating a thermoplastic resin in the production of an optical fiber cable, in some cases, upon heat treatment, styrene gas or the like is evolved from the tension member, resulting in expansion of the optical fiber cable.
Up to now, any thermoplastic resin satisfying the above requirements for a high curing time, highly strong and highly elastic mechanical properties and low viscosity properties has not been developed.
Extensive and intensive studies have been made with a view to developing an optical fiber having excellent bending properties and using a tension member formed of a glass fiber reinforced resin having improved bending properties and flexibility, which has led to the completion of the invention.